new technologies for the drying of coffee

7/29/2019 New Technologies for the Drying of Coffee

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New Technologies for the Drying of Coffee

(Taken from Hygienic Coffee Drying prepared for this resource byJuarez de Sousa and Consuelo Domenici Roberto, University of Viosa,

Brazil 1 )

1. Introduction:

Due to their high moisture content at harvest, 60 to 70% w.b., natural ripecoffee cherries do not flow easily in handling equipment (e.g. gravity spouts,hoppers, bucket elevators, augers) and conventional mechanical dryers.Conventional drum-type rotary dryers are an alternative, but they are generallycriticised for having air-flow problems and low energy efficiency. As a result of this, the typical coffee drying process in Brazil consists of two different dryingstages.

In the first stage, freshly harvested whole coffee cherries are spread on pavedterraces where they are allowed to dry under the sun until they reach 35 to 30%moisture content, whereas in the second stage the coffee is dried in high-temperature mechanical dryers down to about 13% w.b.

Modifications in the drawing and the handling of the drum-type rotary dryer,accomplished by researchers from Universidade Federal de Viosa (UFV), Brazil,show that the above-mentioned problems may be easily solved, and cantransform the rotary dryer into suitable equipment to work with natural, cherry,coffee.

If the climate is such that dependable sunshine is available during the harvestseason, the drying process may be conducted entirely in the open air. In thiscase, cherries are spread out on the drying terrace after harvesting, and dried toabout 13% moisture content in a single operation. Another alternative is thecomplete drying of whole coffee cherries in a fixed-bed dryer or in the newestsystem for coffee drying, the hybrid terrace. The principal drawback in utilizingconventional mechanical dryers has been the fact of their being designedprimarily to dry products other than coffee, and their relative high cost. Despitethe recent efforts to change this situation most of the commercially availabledryers are still inefficient and have thus far been mainly used in large scaleoperations. Since 1998, the CBP&D-Caf (Consrcio Brasileiro de Pesquisa eDesenvolvimento do Caf), through the Universidade Federal de Viosa, has beendeveloping for the coffee grower drying, and storage technologies adapted forsmall coffee production systems, as outlined below.

Some innovative grain drying methods, generally applied to cereals and oilseeds,have been introduced into coffee drying facilities in the last few years in an effortto increase the drying capacity and the energy efficiency of conventional coffeedrying installations, and at the same time to maintain high coffee quality. Thesemethods include dryeration, reversed-direction-air-flow drying, concurrent-flow

1

The views expressed in this publication are those of the author(s) and do notnecessarily reflect the views of the Food and Agriculture Organization of the UnitedNations.

Page 1 of 17 Good Hygiene Practices along the coffee chain


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and counter-flow dryers. Several other high-temperature, continuous-flow dryingsystems, and combination drying, that have been traditionally used to dry cerealgrains to safe moisture levels for long storage periods, have been adapted to drynatural, washed, and pulped coffee ( descascado ). The main new approaches tothe drying of coffee in Brazil are outlined in the following sections.

2. Coffee Drying in Hybrid Drying Terraces:

2.1 Design and construction of facilities

The hybrid terrace is a conventional coffee drying terrace, adapted to use aventilation system with the drying air heated by a charcoal furnace. It makescoffee drying possible in the total absence of solar radiation, during the night,and on cloudy or rainy days.

In the centre, along the length of the terrace, a sub-surface ventilation duct isinstalled (the main duct) with six exits for heated air. Directly above theventilation ducts a metal aeration duct, with 23% perforated area, is placed. Thecoffee beans are uniformly distributed over the aeration duct to be dried with hotair in the absence of solar radiation. Arranged in this way, the terrace can beused as a conventional terrace in good drying conditions, and the furnaceoperated only in rainy weather, or at night time artificial drying can supplementconventional drying.

For the 150 m 2 drying terrace studied (7,000 litres of coffee beans), the heateddrying air is forced to pass through the coffee layer by a 5 hp centrifugal fancoupled with the furnace (see Fig. 1 below).

Fig. 1: Top and transverse view of the hybrid terrace, module of 150 m 2 ,and detail of the ventilation system.



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2.2 Drying operations using a hybrid terrace

As noted above, due to these changes to the drying terrace, coffee drying can becarried out continuously, using solar energy in sunny days and using heated airduring the night or on rainy days. With these procedures the coffee can be dried

from its initial moisture content (62% w.b.) to a final storage moisture content(12% w.b.) in less than 5 days.

From July to September, in Brazil, two hybrid drying terraces with 150 m 2 (or7,000 litres capacity) were simultaneously tested for coffee drying under twodifferent conditions.

For the first drying procedure (Hybrid Terrace 1), the drying was done using theconventional terrace drying method from 09:00 to 15:00, and using forced airheated by biomass energy from 16:00 to 08:00 the next day. For the seconddrying procedure (Hybrid Terrace 2), the aeration system was turned-on

continuously, 24 hours a day, without the use of solar energy. The coffee layerwas turned after each 3h of drying, to achieve an homogeneous moisturecontent. The same types of coffees were dried on a conventional drying terracebetween 9:00 to 15:00 as mean of comparison.

Coffee drying treatments:

T1 = Pulped cherry coffee (Hybrid Terrace 2) air drying temperature at 60 oCT2 = Pulped cherry coffee (Hybrid Terrace 1) air drying temperature at 60 oCT3 = Natural cherry coffee (Hybrid Terrace 2) air drying temperature at 60 oCT4 = Natural cherry coffee (Hybrid Terrace 1) air drying temperature at 60 oCT5 = Pulped cherry coffee (Hybrid Terrace 2) air drying temperature at 40 oCT6 = Pulped Cherry coffee (Hybrid Terrace 1) air drying temperature at 40 oCT7 = Natural cherry coffee (Hybrid Terrace 2) air drying temperature at 40 oCT8 = Natural cherry coffee (Hybrid Terrace 1) air drying temperature at 40 oC

T9 = Pulped cherry coffee (conventional concrete terrace) comparison test for(T1 and T5)

T10 = Pulped cherry coffee (conventional concrete terrace) comparison test for(T2 and T6)

T11 = Natural cherry coffee (conventional concrete terrace) comparison test for(T3 and T7)

T12 = Natural cherry coffee (conventional concrete terrace) comparison test for(T4 and T8)

The results show, as a function of the average drying temperature, that the timenecessary to dry the coffee from treatments T1, T2, T3 and T4 were shorter thanthe drying times for treatments T5, T6, T7 and T8, independent of the dryingsystem, and of the coffee type.



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ih

e f g

cd

ba

Fig. 2: Setting up and operation of the hybrid drying yard in pictures:

a. General view of the drying terrace before the aeration ducts set up;b. Details of the air exits for the aeration duct;

c & d. Positioning of the ducting;e & f. Heaping the coffee in preparation for drying;

g & h. Covering the plastic cowling for operation under rainy conditions;

i. Operation of dryer under plastic - air flow maintaining positive pressure.



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When comparing the necessary times to dry coffee, it can be seen that thedrying time needed for the hybrid drying terrace is, on average, about 1 / 5 and 1 / 6 of the time needed to dry natural cherry coffee and pulped coffee, respectively,using a conventional drying terrace.

Cherry Coffee

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120

Time (hours)

G r a

i n M

. C

. ( %

w . b . )

H1T40 H2T40 H1T60 H2T60

Fig. 3:Drying curve for natural coffee cherry using terrace drying with heated air aerationsystem H1 (solar + biomass energy) and H2 (only biomass energy)

Pulped Cherry coffee

05

1015202530354045

0 10 20 30 40 50 60 70 80

Time (hours)

G r a

i n M

. C

. ( %

w . b . )

H1T40 H2T40 H1T60 H2T60 Fig. 4: Drying curve for pulped coffee cherry using terrace drying with heated air aeration

system H1 (solar + biomass energy) and H2 (only biomass energy)



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H1T40

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120

Time (hours)

G r a

i n M

. c .

( % w

. b . )

Cherry Pulped Cherry

Fig. 5: Drying curves for pulped cherry and for natural coffee cherry using drying terracewith heated air aeration system H1 (solar + biomass energy)

Table 1: Cup quality and coffee beans characteristics after drying tests:

Treatment Cup quality Type Aspect

HT2 (pulped) 60 oC Only soft 4/5 Good


ConvTD (pulped) Hard 6/7 Good



ConvTD (pulped) Hard 6/7 Good

HT2 (natural) 60 oC Hard 5 Good


ConvTD (natural) Hard/Rioy 6/7 Good



ConvTD (natural) Hard/Rioy 6/7 Good

From the results, the dried coffee beans were considered of excellent quality forthe production area under conditions of terrace drying in the Zona da Mata,Minas Gerais. The cup quality did not differ with the use of the drying systems(Hybrid Terraces 1 or 2) for the same coffee type. The same, however, is nottrue when the drying results from the Hybrid Terrace system are compared with

the drying results from conventional drying terrace.



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3. Flex Dryer:

The flex dryer is a combination of the traditional fixed bed dryer (model: UFV-J2) with a solar collector. The drying air can be heated with energy from thecombustion of firewood, charcoal or gas or with solar energy or with a mixture of

solar energy and combustion energy. For the fact of using different sources of energy, it was named Flex dryer.

Fig. 6a: Flex dryer with traditional roof Fig. 6b: Flex dryer with a solar roof collector

The flex dryer incorporates both the furnace and heat exchanger of a silo dryerwithin a natural convection design and a solar collector, with common ducting.To complete the dryer system, a fan was added for use with deeper coffee layerswhere additional pressure is required to force through the hot air. In this case,the dryer works in traditional fixed bed dryer mode with indirect heating by thefurnace.

Note, that in the absence of electricity, which is frequently the case with ruralelectricity supplies, the drying can continue using the natural convection mode.In this case, the height of the coffee layer should be reduced, or more frequentstirring is needed. Besides forcing the convection in the dryer pre-chamber, thefan is also used to draw air over the roof collector channels and be pre-heatedby solar energy. The use of the complementary solar energy, besides reducingsubstantially the consumption of other fuels, is that it is non-pollutant, and the

roof collector design is just a little more expensive than a standard roof.

Fig. 7: Longitudinal section of a natural convection dryer



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4. Concurrent Flow Drying of Coffee:

With the same objectives, as previously mentioned, in the laboratories of Universidade Federal de Viosa an intermittent concurrent-flows dryer wasdesigned, built and evaluated. Its architecture is similar to the counter-flow

coffee dryer and has the following main characteristics: high thermal efficiency, alow initial cost, simple operation and maintenance.

With the evaluation model it was possible to dry natural coffee from an initialmoisture content of 25% to 11% w.b. in 7, 6, and 5 h for drying airtemperatures of 80, 100, and 120C, with specific energy requirements of 5,700,4,870, and 4,760 kJ kg -1 , respectively.

The dryer was built with an effective height of 4.0 m, capable of holdingapproximately 2,300 kg of natural coffee at 25% w.b. moisture content, andoperating with an airflow rate of 27 m 3 min -1 m -2 . The grain flows by gravity from

a 1.5 m deep holding tank, enters a 0.7 m deep drying chamber, and then atempering section 1.8 m high. The discharge auger located at the bottom of thedryer guarantees a coffee velocity of 3.5 m h -1 , so that the retention time of thecoffee in the drying section is 0.2 h. The unloading device removes the partiallydried coffee from the tempering zone and conveys it back to the holding sectionat the top of the dryer. It must be remembered that due to the evaporativecooling effect at the inlet of the drying chamber, the coffee does not reach thedrying-air temperature.

Each drying test was compared with the traditional cement drying system. Eventhough using higher temperatures than those used for conventional hightemperature coffee dryer, the quality deterioration in the coffee brew was notobserved.

In order to verify the behaviour of the dryer operating under different dryingconditions, drying simulations were made. These demonstrated that themaximum temperature reached by the grain increases with the drying airtemperature, and for the same drying temperature, the higher the grain speed inthe drying chamber, the lower the maximum grain temperature.

5. Combination Concurrent and Counter Current Dryer:

In a effort to reduce specific energy requirements and improve the efficiencyduring the drying of natural coffee, Pinto (1993 see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality )proposed a new dryer design, as shown in (Fig. 8), which combines the counterflows and concurrent flows drying methods, in separate stages, in one singledryer. The inlet drying air is located halfway down the coffee column. Drying inthe first stage is accomplished using the counter flows drying concept, afterwhich the coffee flows directly to the second stage, where it will be dried as in aconcurrent flows dryer. The dryer is fitted with a 60 gravity hopper whichdelivers the coffee to a central point below the tempering section, from whence itis transported to an elevator pit. The length of both the counter flows andconcurrent flows drying sections is 1.1 m, and the dryer is capable of holding4,500 kg of natural coffee at 30% w.b. moisture. Coffee velocity inside the dryer



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content) 11.2 MJ.kg -1 . All of the samples presented the same drink quality (hard)and types 4/5, 5 and 4/5 for Tests 1, 2 and 3 respectively (Silva et al., 2001 -see: Selected bibliography under Section 3, specifically the section on CoffeeProcessing and Quality ).

6. Pneumatic Coffee Dryer:

The fixed bed dryer, a model widely used for pre-drying or coffee drying, doesneed stirring for product homogenization during the coffee drying process. Whenmanually executed, the stirring operation uses great physical effort, mainly inthe beginning of the coffee drying, when the product still has a high moisturecontent. During manual stirring, the fixed bed dryer entails: use of unnecessarylabour, energy loss, and an increase in drying time.

For dryers with mechanical coffee stirring, increasing of the grain mass flow rate

in the drying chamber give, in general, a product with better final quality.However, increasing the grain mass speed will increase the specific energyconsumption because grains that pass by faster will lose less humidity per unit of time.

Besides these aspects, the systems for grain load/unload and stirring may causedifficulties in project installation, and increase maintenance and initial dryer cost.

A simple transport system, with a relatively low cost, is the pneumatictransporter. Used in storage facilities, this type of transporter has its origins inpressure equipment used to load and unload ships with grain. The pneumatictransporter moves grains by the use of high-speed air, through a hermetic pipingsystem. With the pneumatic system the product can be transported in anydirection, including along curved pathways. Another interesting aspect is its usein a fixed installation, which can be built without the need of significant structuralchanges.

Fig. 9: On the left showing the furnace and cyclone;on the right the two resting chambers (with the second mounted on the pyramids)

Considering the advantage of the fixed bed dryer and the simplicity and low costof the pneumatic transport system, a coffee drying system that allows the



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movement of the product (loading, unloading and stirring) and the flow of dryingair, using a single centrifugal fan, powered by a 2 hp electric motor, wasdeveloped and evaluated. With these characteristics, the dryer can deliver up to60 kg of dry parchment coffee per hour.

7. Dryeration:

The dryeration process involves both high temperature drying and aeration of theproduct is one of the most innovative drying methods that has been adapted tocoffee drying systems.

In using the dryeration process, a high temperature drying process is concludedat about two per cent above the target moisture content and conveyed to aholding bin where it will be allowed to temper with no airflow for not less than 6h. The coffee is then transferred to another bin where it will be cooled with low

airflow rates. During the aeration or cooling some drying takes place and theremaining two percentage points of moisture are removed.

Dryeration provides three advantages over traditional coffee drying methods(Loewer et al, 1994; Cloud & Morey, 1980 - see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality ):

Increased dryer capacity; Reduced energy requirement; and Better coffee quality.

The effect of drying-air temperature and a tempering or steeping period on themoisture difference throughout a fixed bed of natural coffee, and energyrequirement during the dryeration process, have already been determined(Cordeiro, 1982 - see: Selected bibliography under Section 3, specifically thesection on Coffee Processing and Quality ).

Table 3 summarizes the mean values of final moisture content differencethroughout a 0.4 m deep coffee bed for three levels of drying-air temperature(50, 60, and 70C), one level of airflow rate (15 m 3 min -1 m -2 ) and three levels of tempering period (0, 6, and 12 h).

The values presented in Table 3 were obtained using natural coffee with 28%w.b. initial moisture content; the high temperature drying was interrupted whenthe moisture content of the coffee beans have reached 13% w.b., and twopercentage points of moisture were removed during aeration or cooling.



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Table 3: Values of final moisture content differences found between samples taken fromthe coffee bed (percentage point moisture, w.b.). Dryeration of a 0.4 m deep fixed-bedof natural coffee harvested at 28% w.b. and dried to 11% w.b. with an airflow rate of 15m 3 min -1 m -2 .

Temperature(C)

Tempering period (h)

0 6 1250 3.2 2.3 1.760 3.6 3.0 2.870 3.8 3.0 2.9

Source (Cordeiro, 1982 - see: Selected bibliography under Section 3, specifically thesection on Coffee Processing and Quality )

From the values presented in Table 3, it can be concluded that drying airtemperature of 50C and 12 h of tempering is the best dryeration treatment for

natural coffee. It results in a final moisture difference of 1.7 percentage points;almost 50% less than the difference obtained with the conventional dryingmethod.

It can be seen that that an airflow rate of 15 m 3 min -1 m -2 significantly exceedsthe values normally encountered in grain dryeration processes. However, coffeestorage bins are generally not equipped with perforated floors. Thus the aim is touse the fixed-bed dryer as a tempering bin altogether, so that the fixed costs arereduced by using the same fan for both high temperature drying and aeration.

8. Combination High-temperature, & Ambient-air Drying:

Combination drying is one of the latest concepts in coffee drying technology. It isthe drying method in which both high temperature processes and natural air orlow temperature drying procedures are combined in an attempt to provide a highquality product as compared to using only high temperature drying.

The high temperature drying stage is used to reduce coffee moisture contentsfrom approximately 60% w.b. to 25% w.b. (or less), so that natural air dryingcan be successfully used to complement the drying by further reducing themoisture content of the product to safe limits (Dalpasquale, 1984 - see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality ).

Unlike the dryeration process, where coffee is allowed to temper in a separatebin for several hours before being transferred to another bin where it will beslowly cooled, in combination drying the hot coffee is transferred directly to thestorage bin where the remaining moisture will be removed using ambient air orlow temperatures.

In combination drying, coffee is dried in the high temperature dryer to amoisture level ten to twelve percentage points above the value for safe storage,with the result that the recommended minimum airflow rates for the aerationstage in combination drying are 15 to 25 times more than the maximum airflowrate used in the cooling stage of the dryeration process (Navarro & Calderon,



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1982 - see: Selected bibliography under Section 3, specifically the section onCoffee Processing and Quality ).

Combination drying provides three advantages over typical high-temperaturehigh-speed drying: increased drying capacity, reduced fuel requirement, and

better coffee quality by reducing the final moisture difference throughout thecoffee bed. As compared to the dryeration process, the main advantage of combination drying is the elimination of the extra handling step associated withthe tempering bin. The main disadvantage of combination drying is therequirement of a relatively high level of capital investment and management incases where it is purchased as a unit because two complete drying systems areincluded.

The usual procedure to dry natural coffee on the farm using the combinationmethod is as follows: ripe cherries which have been strip-picked at moisturecontents ranging from 50 to 65% w.b. are separated from the rest by water

flotation and screening, and then dried in either two or three separate stages.Where three stages are used, the coffee is firstly dried on paved sun dryingterraces until it reaches approximately 35% w.b. Then, the partially dried fruitsare conveyed to a high temperature dryer where their moisture content is furtherreduced to about 25% w.b. In the third stage the coffee is bin-dried toapproximately 13% w.b. moisture using natural air.

In using two stages, the sun drying procedure is eliminated and the moisturecontent at harvest is reduced to approximately 25% w.b. using the hightemperature dryer alone. Combination drying can also be applied to the drying of washed and pulped coffee, whereby the product is delivered directly to the lowtemperature drying bin after the sun drying procedure or the high temperaturedrying method.

A typical ambient-air drying bin used in combination drying for small-scaleoperations is 2.0 m in diameter and has an effective height of 2.0 m, a free

volume of 6.3 m 3, and is capable of holding approximately 2,520 kg of coffee at22% w.b. moisture content. The bin wall consists of one brick layer 0.12 m thickcovered with plastering on both surfaces, and a reflectant paint is applied. Thebin is equipped with a perforated floor for drying-air distribution and a 0.2 msquare basal outlet is also provided. The surface of the coffee bed is generally

open to the atmosphere but shielded from rain by a roof erected over the bin andsupported on a wooden frame. The bins must be built over a concrete floorpreviously made damp proof. A straight bladed, centrifugal type fan driven by a735W, 1730 rpm electric motor is used to force air upward through the coffeebed.

The natural air drying stage in combination drying has recommended airflowrates in the one to one to one-half of the airflow range normally used in the hightemperature drying stage (Cloud & Morey, 1980 - see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality ).Thus, airflow rates varying in the range from 7 to 15 m 3 min -1 m -2 are generally

used. This is enough air to dry relatively wet coffee as a result of the dryingcapability of the natural air. The fan should be started as soon as the first lot of coffee is delivered to the bin, and is switched off during some periods of adverse



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drying weather, or when the mean moisture content of the coffee in the surfacelayer of the bed reaches the target moisture content. For natural air dryingsystems, the total drying time depends on the initial moisture content andprevailing climatic conditions.

The mixing of different lots of coffee of markedly different moisture contents, acommon practice from the farm to the final product processor, should be avoidedwhenever possible. All tests carried out using mixed lots of coffee with differentmoisture contents resulted in brews of poor cupping quality. It is believed thatonly when all coffee has the same or similar moisture contents prior mixing, willit produce a brew with full flavour development.

It is expected that one or a number of bad weather days will not adversely affectthe efficiency of low temperature drying systems. Stopping the fan and waitingfor a more favourable weather would fail to recognize that the first and foremostrequirement in ambient-air drying is to keep the grain cool and prevent heating

(Navarro & Calerdon, 1982 - see: Selected bibliography under Section 3,specifically the section on Coffee Processing and Quality ).

However, it is a matter of common knowledge that keeping coffee at moisturecontents over 15% w.b. for long periods of low-temperature drying or storage isunsafe since mould and, consequently, cup quality deterioration may occur. Butrewetting of dried coffee as a result of continuous fan operation during periods of adverse weather is also a hazardous procedure because the beans may becomepale and bleached out in appearance signifying aroma and flavour deterioration.Therefore, unlimited fan operation during poor drying weather conditions may berecommended as long as the moisture content of coffee is above approximately17% w.b.



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9. Physical Properties and Relationships of Interest:

Notation

grain temperature, C

relative humidity, 0 1.0 bulk density, kg m -3

cp specific heat of coffee, kJ kg -1 C -1

h fg enthalpy of vaporization of water incoffee, kJ kg -1

M moisture content of coffee, decimaldry basis

MR

moisture ratio, (M - M e) / (M 0 - M e)

T air temperature, CTab

s

absolute air temperature, K

t time to dry to MR with drying airtemperature Tabs , h

Sufficese equilibrium valueeq equivalent0 initial value

Appendix - Physical properties

a. Thin layer drying equation. The following equation describes the dryingrate of a thin layer of natural coffee at drying air temperatures ranging from40C to 80C (Arteaga, 1986 - see: Selected bibliography under Section 3,specifically the section on Coffee Processing and Quality ).

MR = exp [-105.756 t eq 0.60564 e (-2751.51 / T abs ) ](1)

b. Equilibrium moisture content. One of the main problems in previous

attempts in setting up mathematical models to simulate coffee drying wasthe lack of a reliable equation for its equilibrium moisture content. Rossi andRoa (1980 - see: Selected bibliography under Section 3, specifically thesection on Coffee Processing and Quality ) presented the following empiricalrelationship for the desorption isotherms for natural coffee:

Me = (15272 - 324782 + 33341 3) exp [(-0.029458

- 0.0016309 - 0.013695 2 + 0.0132050 3) T abs ](2)

The resulting sigmoidal curve obtained from Eqn (2) is too steep for relativehumidities above 60% and it does not fit well most of the experimental datafound in literature. It is believed that many failures to validate coffee drying



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were due to the use of this equation to predict equilibrium moisture content.On the other hand, better overall accuracy is attained using the followingempirical equilibrium moisture content equation proposed by Arteaga (1986- see: Selected bibliography under Section 3, specifically the section onCoffee Processing and Quality ).

Me = 1.1282 [-ln(1 - e ) / (T e + 40.535)]0.5405

(3)

c. Enthalpy of vaporization. The absence of an accurate equilibrium moisturecontent equation for coffee also led to inaccuracies in the development of equations for the enthalpy of vaporization of moisture in coffee beans. Thefollowing equation was developed by Berbert & Queiroz (1991 - see:Selected bibliography under Section 3, specifically the section on CoffeeProcessing and Quality ) and was based on the method of correlating vapourpressure and latent heat data presented by Othmer (1940 - see: Selected

bibliography under Section 3, specifically the section on Coffee Processingand Quality ). It was developed using experimental data from Arteaga (1986- see: Selected bibliography under Section 3, specifically the section onCoffee Processing and Quality ) in the 0.15 decimal d.b. to 0.25 decimal d.b.region.

hfg = (2501 + 1.775 ) [1 + 1.872 exp(-20.601 M)](4)

d. Specific heat. The following equation developed by Villa et al. (1978 - see:Selected bibliography under Section 3, specifically the section on Coffee

Processing and Quality ) represents the specific heat of coffee at constantpressure.

cp = 1.674 + 2.510 [M / (1 + M)](5)

e. Bulk density. The effect of moisture content on bulk density of naturalcoffee is given by the following equation developed by Silva (1991 - see:Selected bibliography under Section 3, specifically the section on CoffeeProcessing and Quality ).

= (396.48 + 224 M) / (1 + M)(6)

In a study conducted by Castro (1991 - see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality ), theestimated bulk density of washed coffee as a function of moisture contentwas defined as in the following equation

= 371.78 + 255.17 M(7)



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new technologies for the drying of coffee

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